Eh, I don't think you'd have to explain that much about Rook Kast. Just briefly make it clear that she had doubts about the empire and Saxon is strong-arming her to stay on his side. Her function in the show is far more defined by being Sabine's mom than by her history with Maul, just like how Saxon is going to be defined by being an Imperial Supercommando (and according to another clip, Imperial Viceroy of Mandalore). The show seems to be doing a decent job of establishing that traditional Mandos don't like the Empire any more than the New Mandalorians would have. Fenn Rau may have been effectively blackmailed into joining the Rebellion, but he and his men clearly love their autonomy and traditions. His men honored Sabine's request for a challenge with Rau, even though she's an enemy combatant found sneaking around their base, at least in part because she challenged their allegiance to Mandalore over the Empire. And Rau meets a Jedi, easily still one of the most high priority targets for the Empire, but Rau respects his attempts at diplomacy and clearly considers his argument for joining the Rebellion. Add in Sabine's own fallout with the Empire, and its easy to see her mother having instilled enough old-school pride into her that the idea of being a collaborator bucking Mando traditions while also being a murderer like Saxon is would be unbearable.